The field of the present invention relates to acoustic noise management or mitigation. In particular, apparatus and methods are described herein for upshifting the frequency of acoustic energy to higher frequencies to reduce various effects of the acoustic energy on its surroundings.
Management or mitigation of acoustic noise is an important issue. Acoustic noise arising from aircraft, ground vehicles, construction or industrial machinery, or other sources can have a variety of deleterious effects on human or animal subjects. Examples of such effects are hearing loss (temporary or permanent), emotional or stress reactions arising from noise exposure, disruption or injury of body organs or components by acoustic body resonances, or degradation of a community environment (e.g., depressed property values near a noise source). Acoustic noise signals can also degrade or damage structural or mechanical components or assemblies (that may or may not be the source of the acoustic noise) by inducing undesirable vibrations or oscillations, particularly if such vibrations are near a natural resonance frequency of the component or assembly. Acoustic noise can also be undesirable in military settings by alerting adversaries to the presence of acoustically noisy military hardware.
As an acoustic signal propagates through the atmosphere, it is attenuated through a variety of mechanisms. That attenuation increases rapidly with increasing acoustic frequency. For example, FIG. 1 shows the atmospheric acoustic absorption coefficient calculated from below 100 Hz up to 1 MHz increasing from about 0.02 dB/100 m at 100 Hz to about 0.6 db/100 m at 1 kHz to about 10 dB/100 m at 10 kHz (at 1 atm, 20° C., and 70% relative humidity). In another example using a calculator available via the Internet, acoustic absorption coefficients of 2.2 dB/km and 105 dB/km were calculated at 400 Hz and 8 kHz, respectively (http://www.csgnetwork.com/atmossndabsorbcalc.html; 1 atm, 20° C., 50% relative humidity). An acoustic signal propagating through 3 km of atmosphere would be attenuated by 6.6 dB at 400 Hz and by 315 dB at 8 kHz.
Similarly, attenuation of acoustic signals by ear-protective gear (e.g., passive earmuffs or foam earplugs) or soundproofing material increases with increasing frequency. FIG. 2A is a table listing acoustic attenuation as a function of acoustic frequency for three commercially available foam earplugs; FIG. 2B is a table listing acoustic attenuation as a function of acoustic frequency for commercially available passive earmuffs; FIG. 2C is a table listing acoustic attenuation as a function of acoustic frequency for commercially available soundproofing material.
Acoustic signals can be directed to propagate as a directional acoustic beam. Diffraction of the acoustic signal causes such a beam to spread as it propagates; that spread can be characterized by a solid angle. The solid angle characterizing a directional acoustic beam is roughly inversely proportional to the product of (i) the area of an aperture, plate, or other structure that emits the acoustic beam and (ii) the square of the frequency of the acoustic signal propagating as the acoustic beam. For a given emitter size, an acoustic signal at a higher frequency can be directed into a tighter acoustic beam. For example, an acoustic signal at 8 kHz can be directed as an acoustic beam that is 400 times narrower (in terms of solid angle) than an acoustic beam carrying an acoustic signal at 400 Hz.